Folate receptor α (FRα), also known as folate binding protein (FBP), LK26 trophoblastic antigen and GP38, is a membrane-bound glycosylphosphatidyl-inositol-linked protein responsible for some folate transport in the various epithelial tissues, the choroid plexus, the retina and the placenta (Weitman et al., Cancer Res 1992; 52:3396-401; Chancy et al., J Biol Chem 2000; 275:20676-84; Solanky et al., Placenta 2010; 31:134-43). Under normal conditions, the FRα isoform is found at low levels in the apical (luminal) cell membrane of various epithelial tissues or located behind the blood-brain or blood-retinal barriers.
In contrast, a large number of primarily epithelial-derived malignancies have been found to highly express FRα. Greater than 90% of epithelial ovarian cancers; 20-50% of breast, colorectal and lung carcinomas (e.g., NSCLC), as well as cervical cancer (Pillae et al., J. Clin. Pathol. 56:569-574, 2003), uterine carcinomas, osteosarcomas, renal cell carcinomas, mesothelioma, CNS sarcomas, ependyomas and pituitary adenomas demonstrate increased expression levels of FRα by IHC, immunoblot or rt-PCR (reviewed by Clifton et al., Human Vaccines 2011; 7(2):183-190).
Further characterization of FRα has revealed that it is functionally important in cancer pathogenesis. Increased expression of FRα in malignancy is associated with increased uptake of folate, a necessary micronutrient for rapidly replicating cells (Antony, Blood 1992; 79:2807-20). Gene transfection of FRα into malignant cell lines causes faster replication (Bottero et al., Cancer Res 1993; 53:5791-6). This has been confirmed on pathogenic analysis of ovarian cancers where FRα overexpression is associated with higher grade, stage and percentage of cells in S-phase (Toffoli et al., Int J Cancer 1997; 74:193-8).
FRα overexpression in ovarian cancer has also been linked to failure to respond to platinum-based chemotherapy and shorter survival in patients with residual disease after primary surgery (Toffoli et al., Int J Cancer 1998; 79:121-6). Similarly, FRα overexpression has been implicated in resistance of melanoma cell lines to methotrexate and colon cancer cell lines to 5-fluorouracil (Sanchez-del-Campo et al., Pigment Cell Melanoma Res 2009; 22:588-600; Liu et al., J Org Chem 2001; 66:5655-63). FRα overexpression is also associated with a worse prognosis in other malignancies, specifically, an increased risk of recurrence (HR: 2.8; 95% CI 1.4-5.6) in breast cancer (Hartmann et al., Int J Cancer 2007; 121:938-42), and a shorter progression-free survival (HR 2.14; 95% CI 1.07-4.28) in uterine adenocarcinoma (Allard et al., Gynecol Oncol 2007; 107:52-7).
The relatively low levels of FRα expression in normal tissue and, often marked overexpression in some malignant tissue make it an attractive target for immunotherapy. Efforts to stimulate passive immunity against FRα-expressing cancer include the use of murine, chimeric and humanized monoclonal antibodies (e.g., farletuzumab), alone or in conjugates to deliver radionuclides, FRα-targeted T cells and stimulatory cytokines to malignant tissues. Additionally, T cell infusion with genetically modified autologous T cells has been used. Active immunotherapy methods for FRα-expressing cancers currently being investigated include the use of folate-localized molecules to enhance cancer immunogenicity, techniques to raise FRα-specific immunity via viral vector, as well as multiple vaccine strategies to include modified whole tumor cells, as well as DNA, dendritic cell and peptide vaccines (reviewed for example by Clifton et al., Human Vaccines 7:183-190, 2011).
Peptide-based vaccines use immunogenic peptides identified from tumor associated antigens. Previous studies have shown that FRα contains naturally processed immunogenic peptides that are recognized by tumor-associated lymphocytes, and that two specific FRα-derived immunogenic peptides, E39 (FRα 191-199) and E41 (FRα 245-253), are capable of enhancing tumor-associated lymphocyte (TAL) proliferation and anti-tumor function (Peoples et al., Ann Surg Oncol 1998; 5:743-50). TALs stimulated with these peptides are capable of recognizing and lysing FRα-expressing tumor cells of various tissue origins (Peoples et al., Clin Cancer Res 1999; 5:4214-23). Furthermore, FRα-antigen reactive lymphocytes have been identified in patients with breast and ovarian cancer and, to a lesser degree, in some healthy individuals (Knutson et al., J Clin Oncol 2006; 24:4254-61; Lee et al., Oncol Rep 2000; 7:455-66; Kim et al., Anticancer Res 1999; 19:2907-16).
U.S. Pat. Nos. 7,547,759, 8,258,261 and 8,815,256 disclose compositions and methods for inducing immunity against a cancer expressing FRα using immunogenic peptides. One of the peptides, E39, has been studied in human clinical trials. E39 was included with four other major histocompatibility (MHC) class I and one MHC class II peptides along with immunoadjuvant in a phase I trial enrolling patients with ovarian and fallopian tube carcinomas. Of the five HLA-A2 patients who received the E39 peptide, two (40%) developed a functional T cell response by ELISpot (Chianese-Bullock et al., J Immunother 2008; 31:420-30).
Despite the myriad of approaches being investigated, durability of response remains limited and it is not clear which, if any, active and passive immunotherapies will ultimately be clinically effective. Accordingly, there is still a continuing need for compositions and methods which deliver an immunoprotective and therapeutic effect for the treatment for FRα-expressing epithelial-derived malignancies.